1. Field of the Invention
The present invention relates to reduced iron briquettes suitable as a stock material of a steel making furnace such as an electric arc furnace.
2. Description of the Related Art
FIG. 5 is a schematic diagram showing equipment for manufacturing reduced iron briquettes. Reduced iron briquettes are generally manufactured by the steps of: feeding reduced iron G prepared in a direct reduction furnace 1 such as a shaft furnace, a fluidized bed furnace, a rotary kiln, or a rotary hearth furnace, to a briquette machine 2 comprising caliber rolls and a breaker, the briquette machine 2 installed continuously from the reduction furnace 1; press-forming the reduced iron G into a sheet having breaking grooves at a predetermined interval; cutting the sheet at the breaking grooves using the breaker; and forming the cut pieces into reduced iron briquettes B1 having a high temperature of approximately 700xc2x0 C. Subsequently, the hot reduced iron briquettes B1 are placed in a quench tank 3 to allow the reduced iron briquettes B1 to be quenched by water inside the tank 3. The quenched reduced iron briquettes B2 are then delivered outside the tank 3 by a conveyor 4.
The manufactured reduced iron briquettes B2 may be immediately shipped to a nearby steel making plant and melted in a steel making furnace. However, most commonly, the reduced iron briquettes are manufactured in countries where fuel price is low and exported to countries in need of an iron source. Accordingly, the reduced iron briquettes are stored and transported several times after being manufactured, including an export process. If the briquettes have low strength, they will suffer from cracking and lose weight due to shattered pieces and fine particles falling off from cracked edges during storage and transportation. Such falling off of fine particles during storage and transportation damages environment and adversely affects transporting vehicles, ships, equipment, and particularly the workers therein. Cracking also causes reoxidation of the reduced iron at the cracked faces, which results in a decrease in metallization and degradation of quality. Cracking, generation of fine particles, and a decrease in metallization cause operational problems such as a decrease in melt yield in steel making plants.
It has been found, as one of the causes of cracking of the reduced iron briquettes, that quenching of hot reduced iron briquettes by placing them in water causes stresses to remain inside the briquettes which generate microscopic cracks and thus makes the resulting briquettes readily breakable even when they are lightly impacted.
Based on this finding, a method for cooling hot reduced iron briquettes by which reduced iron briquettes having a superior anti-cracking property can be manufactured has been invented. This invention is disclosed in Japanese Unexamined Patent Application Publication No. 6-316718 (related art). This related art employs one cooling method selected from: (1) cooling hot reduced iron briquettes simply by spraying water; (2) slow-cooling hot reduced iron briquettes by spraying water to a temperature of 350 to 250xc2x0 C. and then quenching the slow-cooled reduced iron briquettes by placing them in water; and (3) slow-cooling hot reduced iron briquettes by an inert gas or a mixture of air and 20% or more of inert gas, instead of sprayed water, to a temperature of 350 to 250xc2x0 C. and then quenching the slow-cooled reduced iron briquettes by placing them in water.
Since the cooling methods (1) and (2) above use sprayed water to initially cool the hot reduced iron briquette, the surfaces of the reduced iron briquettes are rapidly cooled, resulting in degradation of the strength, although the degradation is not as extensive as when the hot reduced iron briquettes are directly immersed in water. Cooling method (3) does not suffer from degradation in strength due to rapid cooling since the briquettes are initially cooled using gas; however, method (3) requires high cost since expensive inert gas is used to manufacture the reduced iron briquettes, which is a problem.
It is an object of the present invention to provide a method for manufacturing a reduced iron briquette by which degradation in strength of the briquette caused during cooling can be prevented at low costs.
To achieve this end, the present invention provides a method for manufacturing reduced iron briquettes comprising a cooling process for cooling hot reduced iron briquettes to a temperature in a final product temperature range of not more than 120xc2x0 C., the cooling process comprising: a primary cooling step of cooling the hot reduced iron briquettes by steam; a secondary cooling step of cooling the reduced iron briquettes by both steam and sprayed water; and a final cooling step of cooling the reduced iron briquettes by sprayed water to a temperature in the final product temperature range.
Preferably, the cooling rate of the hot reduced iron briquettes is 4.0xc2x0 C./s or less in the primary cooling step and the secondary cooling step, and is 3.5xc2x0 C./s or more in the final cooling step.
Preferably, the hot reduced iron briquettes are cooled from an initial temperature to a temperature in the final product temperature range in 1.5 to 3.0 minutes.
Preferably, the steam generated by the heat exchange between the sprayed water and the reduced iron briquettes during the final cooling step is used in at least one of the primary cooling step and the secondary cooling step.
Preferably, the hot reduced iron briquettes are prepared either by hot-forming a reduced iron material obtained by a direct reduction iron-making process using a briquette machine or by reducing briquette-shaped materials containing iron oxide.
The present invention divides the cooling process of the hot reduced iron briquettes into three steps, and different cooling media are used in each step. For example, only steam is used in one step, both steam and water is used in another step, and only water is used in yet another step. In the present invention, during the primary and secondary cooling steps, relatively moderate cooling is performed. In the final cooling step, the cooling rate is relatively high.
According to the method of the present invention, since gas, i.e., steam, having a temperature higher than water, i.e., approximately 150 to 250xc2x0 C., is used instead of water during the primary cooling step, the temperature difference between the reduced iron briquettes and the steam is significantly smaller than that between the reduced iron briquettes and water. Moreover, unlike water, steam does not absorb heat by evaporation. Accordingly, the surfaces of the reduced iron briquettes are prevented from being quenched and the degradation in strength can be prevented. Furthermore, when reduced iron briquettes are cooled, they are normally stacked in layers, as described below. Since steam is a gas and can enter the gaps between layers more easily than can water, the entire surface of each reduced iron briquette can come into contact with the steam and can be uniformly cooled. Since steam is less expensive than inert gas, the cost required in the method of the present invention is lower compared with method (3) described above where a substantial amount of inert gas is required. Steam has a low oxidizing power compared with air and by itself has substantially the same oxidizing power compared to that of a mixture of air and 20% inert gas. Thus, steam rarely reoxidizes the reduced iron briquettes.
As the reduced iron briquettes are cooled, the temperature difference between the reduced iron briquettes and the stem becomes smaller, thereby decreasing the cooling rate. Accordingly, in the next cooling step, i.e., the secondary cooling step, both steam and sprayed water is used to increase the cooling rate to an extent which does not degrade the strength of the reduced iron briquettes, and to shorten the cooling time.
In the final cooling step, the problem of strength degradation does not occur even when the reduced iron briquettes are cooled at a relatively high cooling rate. Thus, only sprayed water is used to shorten the cooling time.
During the primary and secondary cooling steps, the cooling rate of the reduced iron briquettes is preferably 4.0xc2x0 C./s or less, more preferably, 3.5xc2x0 C./s or less, and most preferably, 3.0xc2x0 C./s or less so as to prevent degradation of the strength. During the final cooling step, the cooling rate is preferably 3.5xc2x0 C./s or more, more preferably, 4.5xc2x0 C./s or more, and most preferably, 5.5xc2x0 C./s or more so as to shorten the cooling time. The cooling rate is controlled by suitably adjusting the temperature and the flow of the steam and/or sprayed water in each of the cooling steps. Since the controllable range of the temperature of the steam and sprayed water is limited, the flow is mainly adjusted to achieve optimum cooling rates.
Preferably, the time required to cool the reduced iron briquette from the initial temperature to a temperature in the final product temperature range, i.e., the cooling time, is 1.5 to 3.0 minutes. A cooling time or less than 1.5 minutes may degrade the strength of the reduced iron briquettes because the cooling rate is excessively high. A cooling time exceeding 3.0 minutes may reoxidize the reduced iron briquettes and decrease the productivity. The cooling time can be adjusted to be within the above-described range by suitably coordinating the cooling rate of each of the cooling steps within the above-described preferable range.
The sprayed water used in the final cooling step exchanges heat with the reduced iron briquettes, evaporates, and becomes steam. This steam may be retrieved and used in the primary and/or secondary cooling step to reduce the amount of steam used in the cooling process. When a sufficiently large amount of steam is retrieved, introduction of steam from an external source such as an additional plant is unnecessary, thereby saving the cost of installing new apparatuses such as a steam generator. Thus, the cost can be further reduced. If the amount of the steam is excessive, the steam may be supplied to other plants.
The hot reduced iron briquettes are not limited to those manufactured by hot-forming reduced iron prepared by a direct reduction furnace using a briquette machine. The hot reduced iron briquettes may be obtained by reducing briquette-shaped material containing iron oxide. For example, the hot reduced iron briquettes may be prepared by: mixing an iron-oxide containing material, an adequate amount of carbonaceous material, and a small amount of binder, if necessary, to prepare a mixture; cold-forming the mixture using a briquette machine into briquettes; and reducing the resulting briquettes by heating in a rotary hearth furnace. It should be noted that the reduced iron prepared by reducing the material iron at a relatively low temperature of 700 to 900xc2x0 C. in a shaft furnace or a fluidized bed furnace has a large number of micro pores therein. Thus, when this reduced iron is cooled using steam or water without having to undergo hot forming, a problem of severe oxidation occurs during cooling. In contrast, because the rotary hearth furnace generally heats the iron at a high temperature of approximately 1,200xc2x0 C. or more, the reduced iron particles are sintered, thereby decreasing number of micro pores and preventing the problem of severe reoxidation.
According to the method for manufacturing reduced iron briquettes of the present invention, the hot reduced iron briquettes can be cooled at low cost, and the manufactured reduced iron briquettes suffer less from cracking during storage and transportation, generate less fine particles due to cracking, and have superior metallization. Adverse affects on transporting vehicles, ships, equipment, and particularly the workers therein caused by falling off of the fine particles during storage and transportation of the reduced iron briquettes can be prevented. Moreover, reoxidation of the reduced iron at the cracked faces can be less since cracking is minimized. Thus, high-quality reduced iron briquettes can be stably manufactured.